1. Field of the Invention
The present invention relates to a process for producing a shaped electroconductive thermoplastic resin composition article. More particularly, the present invention relates to a process for producing a shaped electroconductive thermoplastic resin composition article, which article has a high electroconductivity, a satisfactory mechanical strength, and a high electromagnetic wave-shielding property.
2. Description of the Related Art
It is known that various electronic devices equipped in cars or home electric appliances sometimes radiate high or low frequency waves and that the radiated electromagnetic waves cause undesirable interference with effective radio waves.
For the purpose of preventing this undesirable interference, electronic devices have been covered by a metal plate or box to stop the radiation of electromagnetic waves from the devices.
Recently, the trend has been to replace the metal plate or box with a thermoplastic resin container. In a conventional electromagnetic wave-shielding thermoplastic resin container, the surface of the container is coated with an electroconductive substance to shield the undesirable electromagnetic waves. However, the coating is not satisfactorily durable and is usually expensive.
Accordingly, it has been attempted to blend and disperse an electroconductive filler, for example, carbon black, carbon fibers, metal powder, metal flakes, or metal fibers, into a thermoplastic resin matrix to provide an electroconductive resin composition for the electromagnetic wave-shielding container.
Usually, in the preparation of the electroconductive resin composition, a large amount of the electroconductive filler is used to enhance the electroconductivity and the electromagnetic wave-shielding property of the resultant shielding container. This large amount of electroconductive filler causes the resultant shielding container to exhibit poor mechanical strength and reduced thermoplasticity and shaping property.
In another attempt, metal fibers are blended into a thermoplastic resin matrix to provide an electroconductive resin composition. In the resin composition, the electroconductivity thereof increases with an increase in the length of the metal fibers. However, the excessively long metal fibers frequently result in formation of undesirable fiber balls in the resultant composition and in difficulty in uniformly dispersing the metal fibers in the resin matrix. Also, the excessively long metal fibers are easily broken while the resin composition is produced by a kneading procedure. The breakage of the long metal fibers results in reduced electroconductivity of the resultant resin composition and in a poor electromagnetic wave-shielding property of the resultant shielding container.
In still another attempt, Japanese Unexamined Patent Publication (Kokai) No. 58-176220 discloses a process for producing an electroconductive plastic composition. In this process, a number of metal fiber bundles having a length of 1 to 15 mm are blended with a plastic resin and the blend is kneaded. The kneading operation is carried out in such a manner that the length of the metal fibers is not substantially reduced.
The above-mentioned kneading procedure is, however, unsatisfactory for uniformly distributing the metal fibers in the plastic resin. Therefore, in order to obtain a desired electroconductivity of the resultant composition, a relatively large amount of the metal fibers should be blended with the plastic resin.